Motor drive systems using speed controls of the basic design illustrated in FIG. 1 are in common usage and are successfully applied in many applications. This type of motor drive system uses an AC/DC converter connected via a DC bus to a DC/AC converter. The DC bus includes a DC bus filter capacitor. The system is controlled by a speed control circuit. Even though this configuration has proven reliable and durable, it suffers from the ripple current limitation of the DC bus filter capacitors. In the very act of performing its purpose as a filter, the capacitors conduct the AC portion of the rectified bus current directly across the converter. The ripple current flowing through the DC bus filter capacitors' internal resistance creates heat and raises the internal temperature of the capacitors reducing their life expectancy. Excessive ripple current can cause premature and catastrophic failure of the capacitors.
The main factors affecting the amount of DC bus capacitor ripple voltage are: whether the incoming AC power is single-phase or multi-phase, the voltage and phase balance in a multi-phase system, the incoming AC voltage, frequency and waveform, the actual power delivered by the motor speed control to the load, the amount of impedance (inductance primarily) in the AC lines supplying the motor speed control, and any bus inductance between the rectifier and the bus capacitors.
Of the factors listed above, the only one controllable by the motor speed control is the power delivered to the motor and the load. Since power is speed multiplied by torque, the motor speed control can adjust load power by either adjusting speed or torque.
Most commonly, but not exclusively, a motor drive system will incorporate a diode bridge rectifier consisting of six diodes in a well-known bridge configuration intended to be powered from a three-phase AC electrical source and producing a continuous DC voltage output. Some ripple voltage is also produced as a by-product of the rectification process.
Ideally, the motor drive system's converter section is supplied with balanced symmetrical sinusoidal line-to-line voltages. For practical reasons the converter section and DC bus filter section (including the DC bus filter capacitors) are designed to accommodate a certain amount of imbalance in the voltages and asymmetry in the phases and distortion of the sinusoidal waveform. Nevertheless, any imbalance or asymmetry in the AC input voltages will increase the ripple voltage on the DC bus and so increase the ripple current through the DC bus filter capacitors. This increases their temperature and decreases their lifetime. The extreme limit of imbalance and asymmetry is single-phase operation.
A motor drive system may include a converter section and DC bus filter section designed to be powered from a single-phase AC power supply. However, most integral horsepower motor drive systems in commercial and industrial use are designed to be powered from a three-phase supply. Many of these with suitable de-rating may be operated from a single-phase power supply. A motor drive system designed to be powered from a three-phase AC power supply can generally only provide less than half its rated power when supplied from a single-phase power supply without severely stressing its converter section and DC bus components. The de-rating factor is chosen primarily to keep the ripple on the DC bus capacitors within acceptable limits.
Most motor drive systems designed to be powered from a three-phase AC supply have some means of detecting unacceptable line imbalance and phase-loss. This can be done by directly monitoring the three-phase AC input, or by observing the ripple on the DC bus either before or after any filter reactor. Motor drive systems respond to the detected line imbalance or phase loss by shutting down if the imbalance persists for longer than a preset time.
Many three-phase motor drive systems can be de-rated for single-phase operation. The de-rating factor is usually enforced by reducing the output current capacity of the motor drive system so that only motors of acceptable power rating can be operated from the motor drive system. Even though the stress on the DC bus is primarily a function of output power and input voltage, this approach has merit because motor current and power are related. However, this approach precludes the use of reduced voltage motors whose power output is within the capacity of the motor drive system, but whose current is higher than the de-rated rating.
The present invention is directed to improvements in phase imbalance protection in motor drive systems.